Conventionally, as a solid-state image sensing device used in an image sensing apparatus such as a digital camera, various proposals have been made so as to realize improvement in resolution and downsizing of the image sensing apparatus. One of such solid-state image sensing devices, U.S. Pat. No. 5,965,875 discloses a structure of a MOS type image sensing device capable of simultaneously obtaining respective R, G and B color components from respective pixels. Hereinbelow, the outline of the image sensing apparatus will be described.
FIG. 1 shows the structure of the solid-state image sensing device disclosed in U.S. Pat. No. 5,965,875, in which photodiodes of respective pixels are 3-layer structured diodes formed in a triple well structure. In FIG. 1, reference numeral 100 denotes a p-type silicon substrate (p-substrate); 102, an n-well formed on the silicone substrate 100; 104, a p-well formed on the n-well 102; 106, an n-region; and 108, a photoelectric current sensor having an ammeter 110 to detect a red (R) component current, an ammeter 112 to detect a green (G) component current and an ammeter 114 to detect a blue (B) component current.
As shown in FIG. 1, the photodiodes are formed as pn junction diodes in 3 layers in a depthwise direction by deeply forming n-type layer, p-type layer and n-type layer sequentially diffused from the surface of the p-type silicone substrate in this order. As light incident on the diodes from the surface side has a longer wavelength, the light enters into the diode deeper. As the incident wavelength and an attenuation coefficient show values inherent in silicon, the depth of the pn junction is designed such that the 3-layer structured photodiodes cover the respective wavelength bands (R, G and B) of visible light, and the electric currents are individually detected from the above-described 3-layer photodiode, thereby optical signals having different wavelength bands can be detected.
Further, the obtained 3 signals are subjected to operation processing and color signal separation, thereby an image can be reproduced.
Further, FIG. 2 shows a pixel-portion equivalent circuit utilizing the photodiodes shown in FIG. 1. In this equivalent circuit, electric signals from the respective photodiodes are outputted from 3 source followers.
In the conventional pixel-portion equivalent circuit in FIG. 2, as the 3 photodiodes are serially connected, 2 photodiodes except the R photodiode connected to the GND are influenced by voltages of the respective photodiodes. Further, as the 3 source followers have a threshold value which varies by pixel, fixed pattern noise occurs. Further, as spectral characteristics obtained by calculation from the structure and densities of the respective layers of the photodiodes overlay-formed in the depthwise direction in FIG. 1, are as shown in FIG. 3, spectral characteristics of general primary R, G and B colors cannot be obtained merely by forming the photodiodes having the structure as shown in FIG. 1.